This invention relates to glass structures bearing a thin, functional, inorganic coating (e.g. a coating of tin oxide forming means to promote reflectivity of infra-red light) which structures have improved appearance as a consequence of reduced iridescence historically associated with said thin coatings, and methods for achieving the aforesaid structures.
Glass and other transparent materials can be coated with transparent semiconductor films such as tin oxide, indium oxide or cadmium stannate, in order to reflect infra-red radiation. Such materials are useful in providing windows with enhanced insulating value (lower heat transport) in ovens, architectural windows, etc. Coatings of these same materials also conduct electricity, and are employed as resistance heaters to heat windows in vehicles in order to remove fog or ice.
One objectionable feature of these coated windows is that they show interference colors (iridescence) in reflected light, and, to a lesser extent, in transmitted light. This iridescence has been a serious barrier to widespread use of these coated windows (see, for example, American Insitute of Physics Conference Proceeding No 25, New York, 1975, Page 288.)
In some circumstances, i.e. when the glass is quite dark in tone (say, having a light transmittance of less than about 25%) this iridescence is muted and can be tolerated. However, in most architectural wall and window applications, the iridescent effect normally associated with coatings of less than about 0.75 microns is aesthetically unacceptable to many people (See, for example, U.S. Pat. No. 3,710,074 to Stewart). Little or no success has been achieved in substantially reducing or eliminating the objectionable and apparent iridescence in clear, blue-green and lightly tinted glasses.
Iridescent colors are quite a general phenomenon in transparent films in the thickness range of about 0.1 to 1 micron, especially at thicknesses below about 0.85 microns. Unfortunately, it is precisely this range of thickness which is of practical importance in most commercial applications. Semiconductor coatings thinner than about 0.1 micron do not show interference colors, but such thin coatings have a markedly inferior reflectance of infra-red light, and a markedly reduced capacity to conduct electricity.
Coatings thicker than about 1 micron also do not show visible iridescence, in daylight illumination, but such thick coatings are much more expensive to make, since larger amounts of coatings materials are required, and the time necessary to deposit the coating is correspondingly longer. Furthermore, films thicker than 1 micron have a tendency to show haze, which arises from light scattering from surface irregularities, which are larger on a such film. Also, such films show a greater tendency to crack, under thermal stress, because of differential thermal expansion.
As a result of these technical and economic constraints, almost all present commercial production of such coated glass articles comprise films in the thickness range of about 0.1 to 0.3 microns, which display pronounced iridescent colors. Almost no architectural use of this coated glass is made at present, despite the fact that it would be cost-effective in conserving energy to do so. For example, heat loss by infra-red radiation through the glass areas of a heated building can approximate about one-half of the heat loss through uncoated windows. The presence of iridescent colors on these coated glass products is a major reason for the failure to employ these coatings.